FIG. 1 shows a typical laser printer 10 including a movable photoreceptor 12, typically a revolving drum or cylinder. This drum 12 is made out of a highly photoconductive material that is discharged by light photons. Initially, the photoreceptor drum 12 is given a total positive charge by a charging electrode 14, typically a wire or roller having a current running through it. As the drum 12 revolves, the printer 10 uses a laser unit 16 (such as a laser diode) to shine a laser beam 18 across the surface of the drum 12 to discharge certain points. In this manner, the laser beam 18 “draws” the text and images to be printed as a pattern of electrical discharges (an electrostatic image) on the drum 12. If the laser beam 18 is modulated, resulting variations in charge on the drum 12 will ultimately be translated to proportionate amounts of toner deposited on a sheet of paper 21.
After the laser beam 18 scans the desired electrostatic pattern on the drum 12, the printer 10 uses a toner roller 20 to coat the drum 12 with positively charged toner powder. Since the toner has a positive charge, it clings to the negative discharged areas of the drum 12 that have been scanned by the laser beam, but the toner does not cling to the positively charged “background” of the drum. With the toner pattern affixed to the drum 12, the drum rolls over the sheet of paper 21 traveling below it. Before the paper 21 travels under the drum 12, the paper is given a negative charge that is stronger than the negative charge of the electrostatic image on the drum 12 so that the paper pulls the toner powder away from the drum 12. Finally, the printer 10 passes the paper 21 through a fuser 24, which is typically a pair of heated rollers and as the paper 21 passes through the fuser 24, the loose toner powder on the paper melts, fusing with the fibers in the paper and forming a permanent image on the paper. After the toner on the drum 12 is transferred to the paper 21, the drum surface passes a discharge lamp 22 that generates a bright light that exposes the entire photoreceptor surface of the drum 12, erasing the electrostatic image. The drum surface then passes the charging electrode 14, which reapplies a positive charge to the surface of the drum 12 in anticipation of the laser beam 18 scanning the next image to be printed onto the drum.
FIG. 2 is a functional top view of the printer 10 of FIG. 1 showing a number of dark areas 200 on the drum 12 that represent the negatively discharged areas or dots created by the scanning laser beam 18. FIG. 3 shows a perspective view of portions of the laser printer 10 in FIG. 1 better showing the scanning of the laser beam 18 in a horizontal direction across the drum 12 as indicated by an arrow 300. Image processing circuitry (not shown) controls the laser unit 16 to modulate the laser beam 18 as the beam scans across the drum 12 in the horizontal direction 300 one line at a time. The image processing circuitry controls the laser unit 16 to turn ON and emit a pulse of light for every dot to be printed in a given horizontal line and to turn OFF where no dots are to be printed in the line.
In scanning the laser beam 18 across the drum 12, the laser unit 16 does not actually move the laser beam 18 itself but instead bounces the laser beam 18 off of a movable mirror 26, such as a rotating mirror or an oscillating mirror. As the mirror 26 moves, it reflects the laser beam 18 through a series of lenses (not shown) and onto the drum 12. These lenses change characteristics of the light beam 18 to compensate for image distortion that would otherwise be caused by the varying distance between the mirror 26 and points along the drum 12.
The laser printer 10 is designed to print pages of paper 21 at a fast rate, meaning the mirror 26 must move at a very fast rate to scan the beam in the horizontal direction and drum 12 must rotate at a fast rate to transfer toner deposited on the drum to the paper. As a result, laser printers with this type of architecture have proven to be extremely sensitive to variations in the rotational speed of the drum 12. These variations in speed of the drum 12 appear on the paper 21 as increased or decreased spacing between horizontal scan lines of dots or pixels formed by the horizontally scanning laser beam 18 and visually appear on the paper as bands. This undesirable effect is called “banding” with the visually noticeable bands being referred to as “banding artifacts.”
Banding will now be described in more detail with reference to FIGS. 4-6. FIG. 4 is a diagram of an ideal scan line pattern 400 produced by the laser printer 10 of FIG. 1 where the rotational speed of the photoconductive drum 12 is constant. The scan line pattern 400 is a pattern of horizontal lines that the laser beam 18 scans along the surface of the drum 12. In the example of FIG. 4, the laser beam 18 scans from left to right in generating each scan line 402-406. In the scan line pattern 400, as the laser beam 18 scans each line 402-406 the laser beam is turned ON and OFF or modulated to generate the desired discharge areas or dots in each scan line. A vertical column of dots including dots 408-412 in FIG. 4 is an example of a discharged area generated by the laser beam 18 as the laser beam traverses the scan lines 402-406. The scan line pattern 400 is ideal in that each scan line 402-406 is perfectly positioned adjacent other scan lines so that the dots 408-412 may be formed in the consecutive scan lines to print the desired vertical column of dots and the corresponding text and/or images including this vertical column of dots. A vertical line 414 shown in FIG. 4 extending vertically from the vertical midpoint of scan line 402 and extending to the vertical midpoint of scan line 406 will be discussed in more detail below with reference to FIG. 6.
FIG. 5 is a diagram of scan line pattern 500 produced by the laser printer 10 that includes banding caused by variations in the rotational speed of the photoconductive drum 12. When the drum 12 rotates too fast, a space or gap G between scan lines 502 and 504 occurs and leads to under development or removal of charge on the drum in this gap, which causes light areas on the paper 21. In the example of FIG. 5, the gap G occurs between scan lines 502 and 504 as shown. A vertical column of dots including dots 508-512 being formed in scan lines 502-506 is thus shifted downward by the gap G so that no charge in this vertical column is removed in the gap. Ideally the top portion of the dot 510 would be formed in the gap G just under the dot 508 in the scan line 502, but due to the increase in speed of the drum 12 and resulting gap G there is no discharge in this area. This results in a light horizontal line or “band” between all dots 508 formed in line 502 and dots formed in line 504. Note that the average rotational speed of the drum 12 is correct due to the operation of control circuitry (not shown) that controls a motor (also not shown) that drives the rotation of the drum. As a result, an increase in rotational speed of the drum 12 will necessarily be followed by a period during which the drum slows down, offsetting the increase in velocity so the desired average velocity is realized. As a result of this slowing down of the drum 12, some scan lines 502-506 may overlap. This is shown in FIG. 5 for scan lines 504 and 506 where the bottom of scan line 504 is shown as dotted since the top of scan line 506 overlaps the bottom of scan line 504 due to the drum 12 slowing down. This overlap results in a region 513 being undesirably scanned twice, once during scan line 504 and once during scan line 506. Note that this additional scan of this region during scan line 506 does not affect the discharge of this region since the region will have been completely discharged during scan line 504 so the region is unaffected by scan line 506 since there is no additional charge removed. A vertical line 514 shown in FIG. 5 extending vertically from the vertical midpoint of scan line 502 and extending to the vertical midpoint of scan line 506 will be discussed in more detail below with reference to FIG. 6.
FIG. 6 is a graph showing the variation in the discharge of dots in the vertical columns of dots in the scan line patterns 400 and 500 of FIGS. 4 and 5, respectively. The vertical lines 414 and 514 shown in FIGS. 4 and 5 represent the vertical axis in FIG. 6 and represents the scan line number or physical location in the scan lines in the vertical direction. The scan line number 1 on the vertical axis in FIG. 6 corresponds to the midpoint of the scan lines 402 and 502. The dotted line in the graph of FIG. 6 corresponds to the ideal pattern 400 where consecutive vertically aligned dots 408-412 are being formed in scan lines 402-406. The horizontal axis in the graph represents the discharge of these dots 408-412, with 1 being fully discharged and 0 being not discharged at all so no dot will be formed. Thus, ideally each of the dots being formed in scan lines 402-406 is completely discharged to 1 as shown by the dotted line in FIG. 6. In contrast, where banding occurs there is a gap in this vertical discharge distribution corresponding to the gap G on the drum 12 of FIG. 5. The graph shows that in the gap G, which corresponds approximately to line number 1.5 in the graph, the discharge falls to zero meaning there is no discharge at all. The discharge then increases again to 1 at just before line number 2 due to the dot 510 in the scan line 504 after the gap G. The discontinuity in the discharge graphically shows why banding occurs since where there has been no discharge no toner will be attracted to the drum 12 in this region. It should be noted that the example of FIG. 6 assumes a perfect linear discharge of the drum 12 and a perfectly uniform laser beam 18. The actual discharge pattern would be different due to nonlinear discharge of regions on the drum 12 and due to a nonuniform intensity profile for the laser beam 18, which would typically be a Gaussian intensity distribution. All examples discussed herein assume a perfect discharge and perfectly uniform laser beam 18 to simplify the examples and allow the concept of overlapping scan lines to be more easily described under these ideal operating conditions.
The principle cause of variations in the speed of rotation of the drum 12 and resulting banding is due to gear noise in gears driving the drum. Gear noise results from imperfect spacing of teeth on the gears, variances in flexing of gear teeth, and other intrinsic variations in gear force transfer. Imperfections on the surfaces of the movable mirror 26 and vibration of the laser unit 16 and mirror 26 relative to the drum 12 can also contribute to banding. Accordingly, existing attempts to reduce banding have focused on improving the mechanical components in the printer 10 in attempts to reduce gear noise and to rotate the drum 12 at a more constant velocity. These approaches, however, can add significantly greater expense to the mechanical components of the printer 10 and thus to the overall cost of the printer.
There is a need for reducing banding in a laser printer without adversely affecting the cost of the laser printer.